The present invention relates generally to the preparation of a single crystal silicon rod grown according to the Czochralski method. More particularly, the invention relates to a non-Dash neck method of preparing a single crystal silicon rod, the rod having a short, thick neck which is dislocation-free.
Single crystal silicon, which is the starting material for most semiconductor fabrication processes, is commonly prepared by the Czochralski method. According to this method, polycrystalline silicon ("polysilicon") is charged to a crucible, which is contained within a crystal puller, and melted. A seed crystal, having a diameter which is typically within the range of 8 to 15 mm, is attached to a pull cable positioned above the melt and lowered until it is brought into contact with the molten silicon. A single crystal silicon rod is then grown by slowly pulling the seed crystal up from the surface of the melt.
As crystal growth begins, the seed crystal exists at a much lower temperature than the molten polysilicon. As a result, when the seed crystal comes into contact with the surface of the melt, it experiences a thermal shock. The thermal shock causes dislocations to be formed in the seed crystal. These dislocations are then propagated throughout the growing crystal and will continue to multiply unless they are eliminated in the neck, a region located between the seed crystal and the main body of the single crystal silicon rod.
The conventional method of eliminating these dislocations involves growing a neck which has a small diameter (typically 2 mm to 4 mm) at a high pull rate (as high as 6 mm/min). This method acts to "squeeze" the neck in order to completely eliminate dislocations before growth of the body of the single crystal silicon rod begins. These dislocations are typically eliminated when the neck, also known as a Dash neck, is grown to a length of up to 100 to 150 mm.
Once the dislocations in the neck have been eliminated, the diameter of the neck is slowly increased until the desired diameter of the body of the single crystal silicon rod is achieved. The body is pulled from the melt until most of the polysilicon is depleted. The diameter is then gradually decreased to a point at which the rod may be separated from the crucible and then removed from the crystal puller.
In addition to the process delays involved with the formation of a neck of this length, problems also arise from the fact that the neck is the weakest point of the crystal and yet is responsible for supporting the entire weight of the single crystal silicon rod. Necks having such a small diameter can fracture during crystal growth, causing the body of the crystal to drop into the crucible. The impact of the crystal ingot and the resulting splash of molten polysilicon can destroy the crucible, susceptor and heater, render the polysilicon melt unsalvageable, and present a serious safety hazard. The neck may also fracture during subsequent manipulation of the single crystal silicon rod after the growth process has been completed. As a result of these potential dangers, a conventional 200 mm diameter crystal having a Dash neck is typically grown to a weight of about 100 kilograms or less, in order to minimize the likelihood of neck fractures.
It has been reported that the diameter of the neck is directly related to the weight of the ingot that can be supported. (See, e.g., Kim et al., Journal of Crystal Growth, 100 (1990), pp. 527-28.) Attempts have thus been made to minimize equipment and raw material losses, as well as safety hazards, that may result from neck fractures by increasing the diameter of the neck. For example, Japanese Kokai No. 05-43379 describes a method of eliminating dislocations while forming a neck having a diameter greater than that of a Dash neck. Dislocations are removed as the neck is grown at a rate ranging from 4 mm/min to 6 mm/min and maintained at a constant diameter, ranging from 4.5 mm to 10 mm, for a length ranging from 30 mm to 200 mm. When the neck diameter exceeds 10 mm, however, dislocations are said to be difficult to eliminate.
In contrast, U.S. Pat. No. 5,578,284 describes a process whereby dislocations are removed from a neck having a diameter which exceeds 10 mm. This process employs a pull rate of less than 4 mm/min and requires a neck length of between 120 mm and 180 mm. It is said that for necks of this diameter, a pull rate of less than 4.0 mm/min results in dislocations being annihilated more quickly than they are formed.
Other attempts to reduce neck fractures have focused on providing additional mechanical support for the crystal body. For example, U.S. Pat. No. 5,126,113 describes an apparatus for supporting a single crystal silicon rod as it is grown. Dislocations in the crystal are eliminated by growing a small diameter neck by the Dash method. A large diameter bulge is then grown beneath the Dash neck before the start of the conical section of the crystal body. Mechanical grips engage the recess beneath the bulge to support the body as it is grown. However, when grips such as these are used to hold the crystal, the steady crystal growth operating conditions may be disturbed, which may also cause the Dash neck to fracture.
In view of these developments, a need continues to exist for a process which eliminates dislocations within the neck of a single crystal silicon rod, such that larger diameter, dislocation-free silicon rods may be produced without substantial equipment damage, loss of raw materials, creation of safety hazards, and reduced throughput and yield.